Of course it matters. If it didn't matter to so many people we wouldn't discuss it so much.
The question is: how important is it? Many people, historicists and mythicists alike, Christians and atheists and others, seem to think that nothing could be more important: if it were ever proven that Jesus never existed, they seem to think, it would remake our world, from the ground up, in a flash.
These people who believe that the proof of Jesus' non-existence would be cataclysmic don't seem to me to have thought it through very far.
Let's look at the case of evolution. Since well before Charles Darwin's time, it has been plain that the nature and origin of life on Earth do not correspond at all to Biblical accounts. And yet, this has not led to an overthrow of Judaism and Christianity: on the contrary, fundamentalists still do not believe in evolution, and scientifically-literate practicing Jews and Christians have had remarkably little trouble in convincing themselves that the Bible never said what it clearly says, and that believers never took it as literally as they clearly all did until a few hundred years ago.
It's much harder to prove the absence of an historical person 2000 years ago than to prove his or her historical reality. Then again, never have so many people been so obsessed with one possibly-historical person as there are with Jesus. The amount of attention paid to him isn't comparable to that paid to anyone else. The usual standards of difficulty of proof or disproof may eventually be swamped by this tidal wave of attention. So let us say for the sake of argument that some day soon it will somehow be conclusively proven, as conclusively as it's been proven that Piltdown Man was a hoax, that Jesus of Nazareth was invented whole by Paul of Tarsus or created in hindsight out of unclear memories of John the Baptist or what have you, that the person himself is shown to be as legendary as his miracles and resurrection. What will happen?
Why should we believe that people will suddenly behave differently than they have in the past? The fundamentalists don't believe that evolution happens just because some biologists have explained that it does. Why on Earth would they be more receptive to historians showing that Jesus didn't exist?
And as for the moderate and the politically-progressive, academically-up-to-speed believers: they have quite calmly kept their beliefs in the face of evolutionary theory by maintaining that the stories in Genesis of the Creation and the Flood are metaphors, or camp-fire stories handed down from generation to generation and never taken all that seriously until all of a suddenly in 19th-century America the Biblical literalsts somehow very suddenly got it all wrong. Why should we expect a smaller amount of faith-saving mental acrobatics if and when Jesus is proven never to have existed? They will tell themselves and each other that Jesus was understood to be just a story all along -- or whatever else they have to tell themselves in order to be able to continue to believe whatever it is that they want to believe.
Although the progressive believers for the most part, and very, very nearly 100% of the academic Biblical scholars and Christian theologians, still firmly maintain that Jesus existed, even the ones who say that all the stories of miracles are legendary (and often have varying absurd positions about no-one ever having really believed those stories anyway -- until the 19th century in the US when millions of fundamentalists somehow managed to pull firm literalist beliefs out of their butts all at once), even the ones who don't believe in God -- although these latter ones have been much more reluctant to call themselves what they are -- atheists -- since Richard Dawkins started to behave like a jackass and put all this stink on the term "atheist" -- although even these Christians, or "2/3 Christians," as Nietzsche called them, still say that they are quite certain that Jesus existed, and tend to rather impolitely mock all doubts -- even they seem to be starting to hedge their bets a little, as more and more of the stories of the early Christians are proven to be legends, and are beginning to say that it doesn't MATTER whether Jesus existed or not, that what matters is the allegorical worth of the stories about him.
In short, although they still firmly maintain that there is no doubt that Jesus existed, they are already laying the ideological groundwork for the case that they may be proven wrong, by emphasizing more and more that "it doesn't matter" if he existed: what matters is the symbolic worth -- whatever that worth might be; it tends to vary quite a bit from progressive theologian to progressive theologian -- of the legendary stories about him. Christians suddenly switched from believing the stories in the Bible literally to maintaining that no-one ever believed them literally. If they could do that, why would it be at all difficult to suddenly switch from saying that it's certain that Jesus existed to saying everyone understood all along that he was a fictional character in those parables known as the Gospels, and that only those pesky fundamentalists, since just very recently, had ever believed in anything like "Gospel truth."
Monday: "Oceania has always been at war with Eurasia and at peace with Eastasia." Tuesday: "Oceania has always been at war with Eastasia and at peace with Eurasia." And only an occasional Winston Smith among the progressive faithful has any qualms or doubts that all is well. That sniveling creep and snitch George Orwell may have fooled millions of readers into thinking that his novels were realistic depictions of Communist regimes, but the Soviet Union actually never was much like that. The parallels between 1984 and Christianity, on the other hand, are many and striking, although, astonishingly, apparently quite unintentional on Orwell's part.
Many naive New Atheists seem to believe that if can be proven that Jesus never existed, superstition will shatter and crash to the ground and a Golden Age of Reason will begin, their descriptions of which sound very much like their families' descriptions of the Millennium. The New Atheist apple has often landed not nearly as far from the fundamentalist tree as it thinks.
I tend to agree, upon reflection, with those who say that it would make little change in the world, at least not right away. I just wish they wouldn't constantly interrupt discussions of Jesus' historicity to say that they don't care about it, because I do care -- not for the sake of huge sudden changes in the world which I don't see coming. I personally am interested in the question of the historical Jesus the same way that I would be interested in any other historical question. I'm interested in history, in trying to determine what happened at such and such a time in such and such a place, for its own sake, in somewhat the same way that theoretical mathematicians enjoy their equations and formulae for their own sake. I just find it interesting to attempt to learn what happened. And if possible to improve the historical record, to make it more accurate and/or more detailed. Some people like detective novels, some like comic books, some like quilts, some like boats. I like old manuscripts and inscriptions and mosaics and other ancient artifacts, and they happen to be what may possibly eventually clarify the historical Jesus question.